MIE TALK – February 2017

Compiled by

Pieter H. Coetzee

Registered person

Introduction

Fire and explosions are a major threat to life and asset integrity in hydrocarbon production facilities as well as in mines. For any explosion to occur there must be an associated ignition source present during a flammable atmosphere release

If a fault develops in a piece of electrical equipment located in an area where flammable gases are present, the fault could cause a disaster if the equipment is not Ex certified that can be used in explosive atmospheres. Flame proof enclosures are some of the protection types used in hazardous locations to prevent explosions and fires. It is important that end users fully understand what they are allowed to do with that Flame proof (Ex d) enclosure so as not to invalidate the certification of the complete equipment.

Legislation

Electrical Machinery Regulation 9(2)

No person may use electrical machinery in locations where there is danger of fire or explosion owing to the presence, occurrence or development of explosive or flammable articles, or where explosive articles are manufactured, handled or stored, unless such electrical machinery, with regard to its construction relating to the classification of the hazardous locations in which it is to be used, meets the requirements of the safety standard incorporated for this purpose in these Regulations under section 44 of the Act.

The following requirements are non-negotiable:

1 Equipment is appropriate to EPL - Gb or Gc /Zone - 1 or 2 requirement of the location – Ex d

2 Equipment group is correct – IIA; IIB; IIC

3 Equipment temperature class is correct – T1 toT6

Note: Remember selection of Explosion Protected Equipment will always follow the area classification.

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What is flame proof equipment and where can I use it?

Flame proof equipment is the type of protection in which the parts which could ignite an explosive atmosphere are located inside an enclosure which can withstand the pressure of an explosion of the explosive mixture inside, and prevents the transmission of the explosion to the explosive atmosphere surrounding the enclosure. Technically unavoidable gaps are so long and narrow that hot gases jetting out will have lost their power to cause ignition by the time they reach the outside of the enclosure.

All three components can be present at flameproof equipment:

EPL: Gb & Gc

Applicable standards: SANS 60079-1

Table 1: Classification of equipment for use in hazardous locations

1 2

Hazardous zone

Equipment classification

0

Present continuously or for long periods

1000hrs + per year

EPL: Ga

Intrinsically safe apparatus of category "ia" (Ex ia) with overvoltage surge protection compliant with an approved standard (note 1).

1

Could occur periodically during normal operation

10-1000 hrs per year

EPL: Gb

Intrinsically safe equipment of category "ia" or “ib” (Ex ia or Ex ib) (note 1) Flameproof (Ex d) equipment Specially protected (Ex s) equipment Increased safety (Ex e) equipment (note 2) Encapsulated (Ex m) equipment

2

Present during abnormal operation, short period’s

0.1-10hrs per year

EPL: Gc

Any type of equipment suitable for zone 0 or zone1

Intrinsically safe apparatus of category "ic" (Ex ic)

Non-sparking (Ex nA) or (ec) equipment

Note: Flame proof equipment is suitable for Zone 1 and Zone 2 locations

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NOTE 1 All intrinsically safe circuits (Ex i circuits) should have system certification from an approved

testing/certification body to ensure that the various devices in the circuit, with the interconnecting wiring and

cabling, are compatible and suitable for the application.

The following information should be submitted for certification of Ex i circuits:

a) a loop diagram showing all equipment and cabling or wiring;

b) information about the equipment characteristics indicated on the loop diagram and in the form of a certificate by an approved testing/certification body; and

c) cabling and wiring information indicating cable types and lengths as well as resistance,

capacitance and inductance characteristics. NOTE 2 Ex e motors should not be used in hazardous locations unless approved overload protective devices are used with such equipment and precautions taken to ensure that such protection cannot be rendered ineffective during use.

NOTE 3 Equipment ratings should be taken into account so as not to cause overheating that leads to

excessive surface temperatures.

Marking of flame proof equipment

Flame proof equipment can be identified by the following marking:

Construction of flame proof equipment

In order to certify flame proof enclosures, the enclosure must be strong enough not to fracture or distort under the internal pressures generated in order to contain explosions and prevent propagation. When testing a product for Explosion proof protection, it will have to undergo an actual explosion within simulated environments at a SANAS Accredited Test Laboratory (ATL).

Figure 1:Flame proof enclosure

Equipment not Ex certified

Flame path

Flame gap

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Characteristics:

Use non certified equipment inside enclosure – can cause a spark.

Explosions may occur inside enclosure

Enclosure contains the explosion.

Withstand the pressure during internal explosion – build strong enough

Flame path prevent that hot gas or flames exit the enclosure prevent ignition of the external atmosphere.

Temperature class:

Measure on outside of equipment.

Different types of Flame proof joints:

The flame path as well as the flame gap of any flameproof equipment are very important and must be well maintained. Damage to flame paths and flame gaps can cause fires and explosions.

The first requirement is that the joint must be robust and be of an approved design.

Secondly, the joints must absorb the heat of the flame or hot gas of an explosion, as it escapes from the enclosure and thereby preventing it from igniting the surrounding atmosphere.

Thirdly, joint surfaces must mate as closely as the method of manufacture allows. The maximum gap allowed for Class A enclosures is 0,5 mm provided that the joint is at least 25 mm wide.

A complete table of flameproof paths and permissible gaps or clearances is given in “Standard Specifications for Enclosures for Electrical Apparatus” (SANS 60079-1)

Maximum experimental safe gap (MESG) Definition: The maximum gap of the joint between the two parts of the interior chamber of a test apparatus which, under specified conditions, when the internal gas mixture is ignited, prevents ignition of the external gas mixture by flame propagation through a joint measuring 25 mm in length.

Table 2: MESG

NOTE: Internal fire or explosion must be “contained” such that it does not ignite the surrounding hazardous atmosphere.

There are four types of approved joints in flameproof enclosures, namely

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Internal flange joints,

External flange joints

Spigot joints

Screwed joints

Figure 2: Internal flange joints Figure 3: External flange joints

Figure 4: Spigot joints Figure 5: Screwed joints

Installations methods of Ex d enclosures

In principle there are three methods of providing electrical connections:

Cables system with indirect cable entry, through separate Connection Boxes

Cables system with direct entry

Conduit Systems

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Setting compound to fill stopping box

Figure 6: Cable system with indirect entry;

Ex line bushings are used to electrically connect equipment in potentially explosive

atmospheres. The connection is established either between a flameproof enclosure and an

enclosure of another type of protection and in accordance with SANS 60079-0.

Combination of Exd and Ex e enclosure (figure 6)

Figure 7: Cable system with direct entry

Appropriated glands must be used – Ex d or a barrier gland as required by SANS 60079-14.

Figure 8: Conduit system with stopping box and setting compound

Ex certified Line bushing

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A sealing device such as a stopping box with setting compound shall be provided, either as

part of the flameproof enclosure or immediately at the entrance thereto.

Direct conduit or tubing entry is not permitted on any Ex d enclosure

Maintenance of joint surfaces

Although faces are machined flat, there will be a slight gap between the flanges when they are bolted together.

Rust on the surfaces of the flanges

When approved flameproof enclosures are used for a considerable period of time where the

conditions are moist, rust may form on the surfaces of the flanges. This rust must not be

removed with any sharp tools, sandpaper or files which can increase the size of the flame

gap. Only a fine steel brush should be used.

No copper coating or any metal inhibited (impregnated) grease should ever be used to

prevent rust between two opposing faces of any approved flameproof enclosure.

It has been proved that copper coating or similar materials diminished the cooling effect of the

flame path.

Prevention of rust between opposite faces

To prevent rust between opposing faces a very thin layer of non-flammable grease can be

used. No solid setting sealant (Silicon rubber) may be used on flameproof joints. A thin layer

of petroleum jelly is normally used.

After maintenance a feeler gauge should be used to make sure that the gap is still within the

specified limits.

Procedure if heat damage is suspected

If it is suspected that heat damage has occurred, flanges should be checked on a surface

table or with a straight edge.

Figure 9: Checking flange faces with a straight edge

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Damage to flameproof enclosure during operation

In the event of a flameproof enclosure or cover becoming cracked during operation, the cover

or enclosure must be replaced. Welding is not permitted on flameproof covers or enclosures,

since the metal normally warps when welded.

Note: If the gap between the flanges becomes too large, the respective flange faces must be re-machined. However any machining which might affect the structural strength of the enclosure must be followed by a hydraulic overpressure test as per SANS 60079-1.

Types of bolts approved for securing joints

The requirements for bolts, screws and studs used in the construction of a flameproof enclosure.

Figure 10: Bolt used in flame proof enclosure

All bolts, screws and studs used in the construction of a flameproof enclosure must meet the following requirements:

Have non-slotted heads – bolts cannot be tightened properly with a screwdriver.

Be of the correct length, nominal diameter and thread form for the tapped hole because it is possible that the wrong length, diameter or thread form bolt could be used in a hole, which may affect the compliance of the enclosure.

Wrong thread form on a bolt may strip threads in a hole. Wrong length of bolt may either have too few threads engaged or if too long could bottom in the hole, thus preventing the cover to be pulled down properly or perhaps pushing a hole into the casing.

Be present and tight – this is the most important requirement to make any flameproof enclosure flameproof, because if all bolts are not tight and present, the specified air gap may be increased during an internal explosion. Every time a flameproof enclosure is assembled, the bolts and studs and holes should be inspected for damaged threads.

Remove any dirt from the bottom of a blind hole before refitting bolts, otherwise the bolt may

bottom in the hole, thereby not correctly securing the cover or perhaps pushing a hole into the

enclosure.

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Factors to consider when installing in hazardous areas:

Problems with cable construction

Figure 11: Cable: Non-hygroscopic/not filled and Hygroscopic/filled

Why use a Barrier Gland ?

Cables are hygroscopic instead of non-hygroscopic.

A Barrier gland must be used where the construction of a Cable is such that a standard Exd

gland with seals will not adequately maintain the integrity of the Exd equipment.

A Barrier gland stops gas and/or explosions bypassing the inner elastomeric seal of a gland

and propagating down the interstices of the cable.

Figure 12: Cable (Unround / filled)

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Figure 13: Explosive environment transfer through cable

Dangers of gas propagation from a hazardous area to a safe area through unfilled cable:

Glands:

Only Ex d or flameproof glands are allowed for Gas groups IIA, IIB and IIC gasses in Zone 2

locations if the cable meets the requirements according to the standards. This also applies for

fire damp in mines.

No direct entry of an Ex d gland is permitted in a Zone 1 location with Gas Group IIC present

as well as IIA and IIB Gasses under certain conditions.(The Ex de combination must be

applied as per figure 6).

Indirect Cable Entry into Ex d Enclosures by making use of Ex e and Ex d enclosures

The two enclosures are connect by certified bushings

Ex line bushings are used to electrically connect equipment in potentially explosive

atmospheres. The connection is established either between a flameproof enclosure and an

enclosure of another type of protection and in accordance with SANS 60079-0.

Types of Ex d enclosures:

Luminaires

Motors

Actuators

Control stations

Common drawbacks

Knowledge of relevant standards and education are fundamental. Mistakes occur due to lack of knowledge or inexperience. The most common mistakes by end users are:

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Many end users replace faulty components with smaller components that will increase the

volume of the enclosure. The results of the flame transmission test performed during the test

laboratory type examination procedure could be negatively affected and a possible internal

ignition will not be safely controlled by the existing flameproof enclosure from the external

explosive atmosphere.

Drilling new cable gland entries into an Ex d enclosure after or prior to installation.

Fit additional components into the enclosure such as a switch or a component. This type of

modification can have an impact on the temperature class rating of the equipment’s enclosure

and can become a source of ignition.

Modifying an Ex d enclosures include accidentally damaging the flameproof gap where the

end user scratch or damage the flanges.

Figure 14: Damaged flame path

The certification of the equipment is no longer valid if any modification is carried by the end user Therefore, if any modifications are made to an Ex d enclosure and these are not within the responsibility and agreement of the equipment manufacturer, only two options exist:

Ex d enclosure must be send back to the OEM for recertification by an ATL.

The end user takes full responsibility which means they also become responsible for any

subsequent failures of the enclosure that could lead to serious health and safety risks.

The relevant standard to consult is IEC/SANS 60079-19 that provides end users with technical instructions on the repair, overhaul and modification of equipment designed for use in explosive atmospheres.

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Painting of flame proof enclosures Painting an Ex d enclosure can cause the following problems:

paint gets in and around the flame gap, can adversely affect the certification

screws that are painted over can prevent access to the enclosure

paint is typically non-conductive that can also lead to electrostatic hazards and introducing an

ignition source .

Objectives of Ex inspection

To confirm the integrity and properties of Ex equipments in the Hazardous area.

To ensure the suitability of Ex equipments in the particular Hazardous location.

To meet national standards, regulations and company safety guidelines.

Types of inspection

Initial Inspection is necessary to check that the selected type of protection is appropriate for the

hazardous area installation before commissioning.

Periodic Inspection is a routine inspection that applies to existing installations. It monitors the effects

of deterioration or change. The intervals between periodic inspections should not exceed two years

Conclusion

All employees working in hazardous locations should first ensure that they understand and recognise

the relevant standards and read the instruction manual provided by the manufacturer before starting

any work on an Ex d enclosure.